There is much interest in the medical uses of nucleic acids. For example, antisense, ribozymes, aptamer and RNA interference (RNAi) technologies are all being developed for potential therapeutic applications. The design of nucleic acids, particularly oligonucleotides, for in vivo delivery requires consideration of various factors including binding strength, target specificity, serum stability, resistance to nucleases and cellular uptake. A number of approaches have been proposed in order to produce oligonucleotides that have characteristics suitable for in vivo use, such as modified backbone chemistry, formulation in delivery vehicles and conjugation to various other moieties. Therapeutic oligonucleotides with characteristics suitable for systemic delivery would be particularly beneficial.
Oligonucleotides with modified chemical backbones are reviewed in Micklefield, Backbone modification of nucleic acids: synthesis, structure and therapeutic applications, Curr. Med. Chem., 8 (10):1157-79, 2001 and Lyer et al., Modified oligonucleotides-synthesis, properties and applications, Curr. Opin. Mol. Ther., 1 (3): 344-358, 1999.
Examples of modified backbone chemistries include:                peptide nucleic acids (PNAs) (see Nielsen, Methods Mol. Biol., 208:3-26, 2002),        locked nucleic acids (LNAs) (see Peterson & Wengel, Trends Biotechnol., 21 (2):74-81, 2003),        phosphorothioates (see Eckstein, Antisense Nucleic Acid Drug Dev., 10 (2):117-21, 2000),        methylphosphonates (see Thiviyanathan et al., Biochemistry, 41 (3):827-38, 2002),        phosphoramidates (see Gryaznov, Biochem. Biophys. Acta, 1489 (1):131-40, 1999; Pruzan et al., Nucleic Acids Res., 30 (2):559-68, 2002), and        thiophosphoramidates (see Gryaznov et al., Nucleosides Nucleotides Nucleic Acids, 20 (4-7):401-10, 2001; Herbert et al., Oncogene, 21 (4):638-42, 2002).        
Each of these types of oligonucleotides has reported advantages and disadvantages. For example, peptide nucleic acids (PNAs) display good nuclease resistance and binding strength, but have reduced cellular uptake in test cultures; phosphorothioates display good nuclease resistance and solubility, but are typically synthesized as P-chiral mixtures and display several sequence-nonspecific biological effects; methylphosphonates display good nuclease resistance and cellular uptake, but are also typically synthesized as P-chiral mixtures and have reduced duplex stability. The N3′→P5′ phosphoramidate intersubunit linkages are reported to display favorable binding properties, nuclease resistance, and solubility (Gryaznov and Letsinger, Nucleic Acids Research, 20:3403-3409, 1992; Chen et al., Nucleic Acids Research, 23:2661-2668, 1995; Gryaznov et al., Proc. Natl. Acad. Sci., 92:5798-5802, 1995; Skorski et al., Proc. Natl. Acad. Sci., 94:3966-3971, 1997). However, they also show increased acid lability relative to the natural phosphodiester counterparts (Gryaznov et al., Nucleic Acids Research, 24:1508-1514, 1996). Acid stability of an oligonucleotide is an important quality given the desire to use oligonucleotide agents as oral therapeutics. The addition of a sulfur atom to the backbone in N3′→P5′ thiophosphoramidate oligonucleotides provides enhanced acid stability.
As with many other therapeutic compounds, the polyanionic nature of oligonucleotides reduces the ability of the compound to cross lipid membranes, limiting the efficiency of cellular uptake. Various solutions have been proposed for increasing the cellular uptake of therapeutic agents, including formulation in liposomes (for reviews, see Pedroso de Lima et al., Curr Med Chem, 10 (14):1221-1231, 2003 and Miller, Curr Med Chem., 10 (14):1195-211, 2003) and conjugation with a lipophilic moiety. Examples of the latter approach include: U.S. Pat. No. 5,411,947 (Method of converting a drug to an orally available form by covalently bonding a lipid to the drug) and U.S. Pat. No. 6,448,392 (Lipid derivatives of antiviral nucleosides: liposomal incorporation and method of use).
Throughout this specification, various patents, patent applications and other types of publications (e.g., journal articles) are referenced. The disclosure of all patents, patent applications, and publications cited herein are hereby incorporated by reference in their entirety for all purposes.